HER2 Testing of Multifocal Invasive Breast Carcinoma How Many Blocks Are Enough

Objectives: To determine the diagnostic yield of testing multiple blocks for HER2 in cases of multifocal breast carcinoma. Methods: We identified 246 consecutive cases of multifocal invasive breast carcinoma in which HER2 was tested on more than 1 tumor focus. We performed an audit of all cases with respect to tumor size, grade, and histologic type. Results: HER2 status was concordant between multiple foci in 230 (93.5%) of 246 cases, with the largest focus having the most positive HER2 result in 242 (98.4%) of 246 cases. We did not find a single case in which a smaller focus demonstrated a more positive HER2 status unless this focus was either higher grade or different histologically. Conclusions: Our findings support the evaluation of HER2 on the largest focus, with additional testing on smaller foci having a different histologic type or higher grade.